a) Field of the Invention
The present invention relates to urethane resins each of which is obtained by reacting an isocyanate compound with an active hydrogen compound.
More specifically, this invention pertains to optical urethane resins which are colored less, have a high total light transmittance and are free of optical strain. The present invention also relates to plastic lenses which comprise the above resins and satisfy the requirement for colorlessness, transparency and uniformity.
b) Description of the Related Art
The present assignee has already filed many applications on plastic lens resins obtained individually by reacting an isocyanate compound with an active hydrogen compound, lenses comprising the same and production processes thereof (Japanese Patent Laid-Open Nos. 295201/ 1989, 302202/1989, 153302/1990, 02/1990, 802/1990, 167330/1990, etc.). Isocyanate compounds useful in the above inventions are, however, accompanied by the drawback that, because of the high reactivity exhibited by each isocyanato group thereof, they lack stability and tend to develop a color and/or, due to self-polymerization, turbidity during storage. Addition of a stabilizer is therefore indispensable to suppress coloration and self-polymerization.
Various compounds have heretofore been known as stabilizers for isocyanate compounds. Representative ones among them are phenolic compounds such as 2,6-di(tertiary-butyl)-p-cresol (U.S. Pat. No. 3,715,381) and phosphite esters such as triphenyl phosphite (Japanese Patent Publication No. 33438/1970).
In addition, many other stabilizers have also been known, including ureas, carbamates, acid amides (Japanese Patent Publication No. 7,044/1970, Japanese Patent Laid-Open No. 36,546/1975), acid substances such as perchloric acid and trifluoromethanesulfonic acid (German Patent No. 2,837,770), carbon dioxide and sulfur dioxide (U.S. Pat. No. 3,247,236), organic amines (Japanese Patent Laid-Open No. 101,344/1975), acid chlorides (Japanese Patent Laid-Open No. 179,917/1988), siloxanes (Belgian Patent No. 858,921), organotin compounds (European Patent Publication No. 203,874), acylisocyanate compounds (Japanese Patent Publication No. 3,825/1973).
Nothing has however been recognized at all regarding possible effects of a stabilizer for an isocyanate compound on the yellowness, total light transmittance and optical isotropy of a urethane resin obtained by reacting the isocyanate compound with an active hydrogen compound.